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    <title>Trim URLs</title>
    <link rel="stylesheet" href="doc.css">
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  <body>
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  <div id=content>
<h1>Trim URLs</h1>
<h2>Configuration</h2>
<p>
The 'Trim URLs' filter is enabled by specifying:
<dl>
  <dt>Apache:<dd><pre class="prettyprint"
     >ModPagespeedEnableFilters trim_urls</pre>
  <dt>Nginx:<dd><pre class="prettyprint"
     >pagespeed EnableFilters trim_urls;</pre>
</dl>
<p>
in the configuration file.
</p>

<h2>Description</h2>
<p>
This filter trims URLs by resolving them by making them relative to the base
URL for the page.  E.g. on <code>http://www.example.com/</code>,
<code>"http://www.example.com/foo"</code> would be shortened to
<code>"foo"</code>.  The filter works only on URLs that are the values
specified by <code>src</code> or <code>href</code> attributes.
It also trims image URLs in CSS if
<a href="filter-css-rewrite"><code>rewrite_css</code></a> is enabled.
</p>
<p>
The filter reduces the transfer size of HTML files by shortening most of the
URLs. Depending on the HTML file, this filter can significantly reduce the
number of bytes transmitted on the network.
</p>

<h2>Operation</h2>
<p>
The 'Trim URLs' filter removes unnecessary bytes on the wire. While it's
useful for development to fully specify your URLs so that links don't break when
things move around, these are bytes that are sent unnecessarily on the wire.
</p>
<p>
For example, if the HTML document looks like this:
</p>
<pre class="prettyprint">
&lt;html&gt;
  &lt;head&gt;
  &lt;base href="http://www.example.com/"&gt;
  &lt;/head&gt;
  &lt;body&gt;
    &lt;a href="http://www.example.com/bar"&gt;Link with long URL&lt;/a&gt;
    &lt;img src="http://www.othersite.example.org/image.jpg"&gt;
  &lt;/body&gt;
&lt;/html&gt;
</pre>
<p>
Then PageSpeed will rewrite it to:
</p>
<pre class="prettyprint">
&lt;html&gt;
  &lt;head&gt;
  &lt;base href="http://www.example.com/"&gt;
  &lt;/head&gt;
  &lt;body&gt;
    &lt;a href="bar"&gt;Link with long URL&lt;/a&gt;
    &lt;img src="//www.othersite.example.org/image.jpg"&gt;
  &lt;/body&gt;
&lt;/html&gt;
</pre>

<h3>Example</h3>
<p>
You can see the filter in action at <code>www.modpagespeed.com</code> on this
<a href="https://www.modpagespeed.com/examples/trim_urls.html?ModPagespeed=on&amp;ModPagespeedFilters=trim_urls">example</a>.
</p>

<h2>Requirements</h2>
<p>
Only URLs referenced by <code>href</code> and <code>src</code> attributes and,
if <a href="filter-css-rewrite"><code>rewrite_css</code></a> is enabled,
URLs in CSS files are rewritten. URLs that occur elsewhere are not altered.
</p>

<h2>Risks</h2>
<p>
  Trimming URLs is considered medium risk. It can cause problems if it uses
  the wrong base URL. This can happen, for example, if you serve HTML that
  will be pasted verbatim into other HTML pages. If URLs are trimmed on the
  first page, they will be incorrect for the page they are inserted into.
  For this reason, it is off by default.
</p>


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